1. Field of the Invention
This invention relates to unipolar, intraband optoelectronic transducers in general and, more particularly, to intersubband (ISB) semiconductor lasers and intraband photodetectors that include micro-cavity resonators.
2. Discussion of the Related Art
Optoelectronic transducers convert optical energy to electrical energy (e.g., a photodetector) or they convert electrical energy to optical energy (e.g., a laser). When the active regions of these devices are made from semiconductor materials, they may be either bipolar (e.g., conventional interband photodiodes or laser diodes) or unipolar (e.g., conventional ISB lasers or intraband quantum well infrared photodetectors, known as QWIPs).
We focus here on unipolar optoelectronic transducers including illustratively both ISB semiconductor lasers and intraband photodetectors.
Within the span of a few years, ISB lasers in general, and quantum cascade (QC) lasers in particular, have established themselves as the leading tunable coherent semiconductor sources in the mid-infrared (mid-IR) and far-infrared (far-IR) range of the electromagnetic spectrum. [See, F. Capasso et al., IEEE J. Quan. Elec., Vol. 38, p. 511 (2002), which is incorporated herein by reference.] Their uniqueness stems from the use of an intraband optical transition: the device is unipolar, and electrons, for example, undergo a quantum jump between quantized conduction band states, called subbands, of a suitably designed semiconductor multi-quantum-well structure. However, due to the naturally transverse magnetic (TM) polarization of the intersubband transitions, QC lasers are intrinsically only in-plane emitters, with the electric-field vector perpendicular to the plane of the semiconductor layers.
Typically, therefore, the cavity resonator of a QC laser has its axis in the plane of the active region. It is formed, for example, by a pair of parallel cleaved crystal facets that act as mirrors. In general, such resonators are relatively long (e.g., 1–3 mm) due to the low reflectivity of the facets.
Surface emission, on the other hand, cannot easily be achieved in QC lasers, although such a characteristic would be extremely desirable for several applications. By surface emission (or vertical emission) we mean emission whose principal direction is transverse to the layers of the device; that is, the optical output emerges through the top or bottom major surface of the device, rather than parallel to the layers of the device; that is, and through the side or edge surfaces of the device. Previous attempts to develop surface-emitting QC lasers have made use of second-order Bragg gratings superimposed on conventional edge-emitting QC lasers, but those designs did not address the large size of the devices. [See, for example, D. Hofstetter et al., Appl. Phys. Lett., Vol. 75, p. 3769 (1999) and W. Schrenk et al., Appl. Phys. Lett., Vol. 77, p. 2086 (2000), which are incorporated herein by reference.]
Thus, a need remains in the art for a surface-emitting ISB laser that is relatively small in size.
In contrast, surface-emitting interband lasers, which are inherently bipolar, are common in the art. One design, known as a VCSEL, includes an active region sandwiched between multilayer dielectric and/or semiconductor mirrors that form an optical cavity resonator. The principal optical output is directed normal to the layers and emerges through either mirror, depending on which one is made to be partially transmissive. [See, for example, L. M. Chirovsky et al., U.S. Pat. No. 6,169,756 issued on Jan. 2, 2001, which is incorporated herein by reference.]
The same principles of physics that render ISB lasers intrinsically in-plane emitters also dictate that these ISB structures would be inefficient detectors of vertical illumination; i.e., light that is incident on the device in a direction that is transverse to the layers of the device.
Similar limitations exist for QWIPs, which are unipolar, intraband devices in which optoelectronic transitions typically take place between a ground state and a continuum (rather than between subbands). See, H. C. Liu, “Quantum Well Infrared Photodetector Physics and Novel Devices”, Ch. 3, pp. 129–196, in Intersubband Transitions in Quantum Wells: Physics and Device Applications I, Eds. H. C. Liu and F. Capasso; Series: Semiconductors and Semimetals, Vol. 62, Academic Press, London, UK (2000), which is incorporated herein by reference.
Thus, a need also remains in the art for a vertically-illuminated unipolar, intraband photodetector.